Forming molecularly oriented containers from reheated preforms

ABSTRACT

In forming containers from injection molded preforms of thermoplastic material such as nitrile-based polymers by a process which includes heating the preforms to orientation temperature followed by distension to container form in a mold, the improvement providing reduced thickness variability in lower portions of the containers which involves controlling preform shrinkage during such heating to between about 4 to 15% of the initial length for nitrile materials by maintaining the ratio of average thickness to inside preform surface area within defined limits and then axially and radially stretching such preforms during distension to predetermined levels. Nitrile preforms convertible to such improved quality containers have values of between about 0.005 to 0.011 inch -1  for the aforementioned ratio.

CROSS REFERENCE TO RELATED COMMONLY OWNED APPLICATION

U.S. Serial No. 790,692, Filed Apr. 15, 1977

BACKGROUND OF THE INVENTION

This invention relates to forming molecularly oriented containers ofthermoplastic material, and more particularly to improvements in apreform reheat process for forming containers especially of nitrilethermoplastic material where strength is developed and wall thicknessvariation minimized especially in the lower container body portions.

Systems for forming containers from preforms reheated to moldingtemperature and then expanded in a mold are known. If the preform at thetime of reforming is at molecular orientation temperature which isusually just above the glass transition temperature zone for thematerial, the resulting stressed containers have improved impact andburst strength which makes it possible to achieve a significantreduction in weight for a given performance over that required whenforming at higher molding temperatures. As also known, thermoplasticmaterials containing a major proportion of polymerizednitrile-group-containing monomer can be fabricated into orientedcontainers in this manner and, though usable for packaging a widevariety of products such as foods, pharmaceuticals, personal care,household and industrial compositions and the like, in view of theirexceptional strength and barrier properties they are especiallydesirable for packaging pressurized contents such as carbonated liquidsin the form of soft drink beverages and beer.

Preforms of these and similar materials, however, present problems in areheat process in that the temperature range within which orientationcan be developed is quite narrow, as typically exemplified by themodulus-temperature plot of FIG. 6 of U.S. No. 3,814,784, andaccordingly reheat process parameters for such materials must be tightlycontrolled. Consequently, though possible to form oriented high nitrilecontainers via a preform reheat process, it is important in obtaininghigh yields with minimum usage of material to precisely controlvariables such as preform wall thickness and the temperature pattern inthe walls at the time of blowing. In this last respect, heat programmingis usually employed to locally influence the extent of stretch duringcontainer formation. Also, though desirable for control it is difficultand most likely impossible to accurately measure temperature through thethickness of the preform wall after reaching orientation temperaturesince surface deformation will occur if a probe is used and radiationtechniques are only effective to provide surface measurements.

Regarding the manner of forming preforms for such a reheat process,injection molding is preferred to minimize excess wall thicknessvariation since the plastic is molded in a cavity delimited by twosurfaces defining the inside and the outside of the molded partvis-a-vis blowing where the inside surface of the part is not formed toa cavity wall. However, in pumping relatively stiff high nitrilethermoplastic material into an injection mold, frozen strains willinherently develop on cooling. Such strains relieve during reheatresulting in shrinkage along the preform length which has to be dealtwith since no way has yet been found to entirely avoid developing suchstrains in an elongated preform. More specifically, a system employingtemperature programming during reheat typically results in a region ofthe preform exposed to a heat source at one temperature graduallyapproaching the desired level for such region and then, because ofstrain relaxation, retracting to a position where the same plastic whichhad been exposed to the first source is now before a source set at adifferent temperature. When preforms subject to such overlappingexposure are expanded in the mold substantial thickness variabilityresults which in turn can lead to excessively thin or thick areas andthe apparent need for more material in the container than is reallynecessary for the intended end use.

SUMMARY OF INVENTION

Now, however, improvements have been developed which substantiallyminimize or overcome such prior art difficulties in a preform reheatprocess for forming molecularly oriented containers of thermoplasticmaterial.

Accordingly, a principal object of this invention is to provideimprovements in such a reheat process which result in improved controlof material thickness distribution without sacrifice in strength orincrease in material usage in lower portions of the resultingmolecularly oriented containers.

Another object is to recognize preform shrink in such a reheat processas a controllable parameter influencing material thickness distributionin the containers.

Another object is to use preform shrink instead of direct temperaturemeasurement as a hot preform quality control parameter in a reheatsystem for forming molecularly oriented containers.

An additional object is to relate shrink and dimensional characteristicsof an injection molded preform and to control such relationship withindefined limits in producing high yields of finished, molecularlyoriented containers of optimum quality.

A further object is to provide such tubular preforms havingpredetermined dimensional characteristics within defined limits, whichwhen heated and stretched axially and laterally will yield containers ofimproved quality in terms of material thickness variability.

Yet a further object is to utilize high nitrile polymers as thethermoplastic material in carrying out the aforementioned objects.

Other objects of this invention will in part be obvious and will in partappear from the following description and claims.

In broad terms these and other objects are accomplished by providing amethod of forming containers from injection molded preforms ofthermoplastic material which comprises providing preforms havingpredetermined dimensional characteristics such that shrinkage duringheating to orientation temperature is maintainable within predeterminedlimits, heating such preforms to such temperature by temperatureprograming while maintaining shrinkage within such limits and thenaxially and radially stretching such heated preforms beyondpredetermined minimum levels but within predetermined total stretchlevels to form the containers, whereby strength is developed andthickness variability minimized.

In more specific terms, there is provided in the method of formingcontainers from injection molded preforms comprising a major proportionof a polymerized nitrile-group-containing monomer, which method includesheating the preforms to molecular orientation temperature followed byaxial and radial stretching to container form in a mold, the improvementtherein providing reduced thickness variability in lower portions of thecontainers which comprises, in combination, the steps of controllingshrinkage of the preforms during such heating to between about 4 to 15%of the total initial preform length and then controlling the extent ofsuch stretching according to the relation: ##EQU1## wherein:

A is at least about 30;

B is at least about 100; and

A plus B is between about 130 and 280.

From a product standpoint a tubular, injection molded preform comprisinga major proportion of a polymerized nitrile-group-containing monomer isprovided for forming into container shape which has a value of betweenabout 0.005 to 0.011 inch⁻¹ for the ratio:

BRIEF DESCRIPTION OF THE DRAWINGS

In describing the overall invention, reference will be made to theaccompanying drawings wherein:

FIG. 1 is a graphical representation in accordance with an embodiment ofthe invention of the relationship between shrink of a nitrile preformand certain of its dimensional characteristics;

FIG. 2 is a partial, sectional perspective view of a preformconfiguration functional in the process of the invention;

FIG. 3 is a schematic view of the preform heating step in a reheatprocess.

FIG. 4 is a schematic elevational view of a stretch-blow assemblyconverting the preform of FIG. 2 into container form; and

FIG. 5 is a graphical representation of the levels of stretch to be usedwith a nitrile polymer version of the preform of FIG. 2 in formingcontainers according to FIG. 4.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring now to the drawings, an elongated, tubular, injection moldedpreform 10 of thermoplastic material is shown in FIG. 2 which can bedistended into container shape in accordance with the invention. Preform10 is circular in cross section and includes body portion 12 havingclosed end 22 which is shown curved in the shape of a hemisphere butcould be of alternate configuration such as substantially flat, pointed,concave or the like. Annular finish portion 14 preferably surrounds anopposite open end, is formed to final shape in the preform injectionmolding step and is not intended for remolding with body 12 duringformation of the container. Alternatively, such finish may be formedduring the final molding step and in such case the preform duringinjection molding will be provided with a length increment correspondingto 14 from which such finish is formed in the blow mold. The wall ofbody 12 may be substantially constant or progressively graduallyvariable in thickness depending on the nature of the thermoplasticmaterial. Body 12 extends from finish 14 at substantially the samecrosswise dimensions and cross sectional shape as that of portion 14 andthrough such body may vary from this configuration it should not beappreciably greater in such dimensions or different in shape than finish14. With nitrile polymers body 12 preferably smoothly increase inthickness along its length from a minimum immediately adjacent thefinish to a maximum at the junction of the sidewall with hemisphericalend 22. The sidewall of body 12 also preferably tapers inwardly at aslight angle θ on the order of about 1/4 to 3/4 degrees to facilitateextraction from the injection mold. Preforms 10 have predetermineddimensional characteristics such that shrinkage during heating in amanner to be described is maintainable within predetermined limits. Morespecifically, the ratio of average preform thickness to the insidesurface area of body 12 lies within a predetermined range to be furtherdescribed, such average thickness being the arithmetical mean of a.) thethickness at 16 in the vicinity of the junction of the body with finish14 and b.) the thickness of the wall at 20 at the confluence of thesidewall with closed end 22.

Containers formable according to the invention from preforms 10 may varywidely in size and shape and can be characterized in terms of weight andvolume as ranging from between about 0.03 to 0.13 gms./cc. of internalvolume. The preferred configuration is a bottle shown as 34 in FIG. 4which is circular in cross section, has a volume of between about 170 to3780 cubic centimeters, has a maximum diameter D somewhere along itslength and a lower body region where wall thickness control for optimumfunctional performance is important. Such area in FIG. 4 is shown asheel or chime 45 in the region of the confluence of the bottle sidewalland base.

Referring to FIG. 3, during temperature programmed heating to molecularorientation temperature each preform 10 while supported on a suitablecarrier, not shown, is interposed between opposing banks 24, 26 ofheating assemblies, each of which comprises a plurality of immediatelyadjacent, vertically arranged emitter strips typically shown as 28, 30,with reflectors 29 interposed therebetween, each pair of opposing stripsbeing in heat transfer proximity opposite a preform zone, with eightzones shown for the particular preform illustrated. The heatingassemblies affecting each zone are set and adjusted to a predeterminedtemperature to accommodate the particular plastic of the preform viaconventional control instrumentation. Such settings are arranged toprovide the specifically desired temperature within the overallmolecular orientation temperature range for the portion of preform body12 in such zone, yet while controlling the amount of shrink via suchsettings which occurs in raising preform 10 to orientation temperaturefrom substantially room temperature. Though a single preform betweenoppositely arranged heating assemblies is shown and preferred to provideoptimum zone temperature control, it is possible to interpose pluralrows between opposing assembly pairs when the nature of thethermoplastic material in terms of modulus change with temperaturewithin the orientation range does not dictate a need for unusuallyprecise temperature control. In the illustrated embodiment, finishportion 14 is vertically below and outside the influence of the heatingassemblies and therefore no increase in temperature to any substantialextent occurs in such finish during heating since reshaping in thecontainer forming step is not contemplated. If reshaping iscontemplated, the portion to form the finish should be within theinfluence of the heat transfer assemblies.

With respect to FIG. 4, a stretch blow assembly 32 is exemplarily shownfor converting a preformm 10 into molecularly oriented container 34.This is accomplished by first enclosing each preform 10 while within themolecular orientation temperature range for the particular thermoplasticmaterial within partible sections 36, 38 of conventional blow mold 40.Next, stretching mechanism 42 is moved over the open end of blow mold 40whereupon telescopic rod 44 is caused to move to extended position by asuitable mechanism, not shown, in order to draw hemispherical end 22against base portion 46 of blow mold 40 thereby axially stretching bodyportion 12 in the manner illustrated in phantom at 48 in FIG. 4.Simultaneously therewith or preferably immediately thereafter, blowingmedium such as compressed air is admitted to the interior of the preformthrough openings 50, 52 in rod 44 to stretch it radially outwardlyagainst the cavity walls to the shape of bottle 34. Under certaincircumstances, for example those contemplating non-pressure applicationsfor the finished container, it may not be necessary to provide aseparate stretch rod in that the pressue of the blowing medium and thereduced length of the preform versus the container may be adequate toprovide the axial stretch desired.

The amount of axial and radial stretch is defined by the configurationof the blow mold cavity in comparison with that of the preform and willvary with the nature of the material involved. In general, if stretch istoo great in one direction there will be significant imbalance oforientation in that direction which results in substantially reducedstrength in the oppsoite direction, whereas if stretch is too low thereverse is true. For example, with excessive axial stretch good columnarstrength is achieve at the expense of hoop strength such that anunwanted hole may develop in the preform during blowing. Such stretchamounts during formation of the container must be greater thanpredetermined minimums but within predetermined total levels. The areawithin the cross hatched portion of the graph of FIG. 5 represenrts theaxial and radial stretch amounts for preforms comprising a 70/30 weightpercent acrylonitrile/styrene polymer which may be successfully empolyedin forming containers according to the invention. The percentage axialand radial stretches as used in FIG. 5 are defined by the formulas:##EQU3##

In accordance with the process of this invention, injection moldedpreforms 10 to be subject to the described heating step are providedwhich have the ratio of average preform thickness to inside body surfacearea within predetermined limits such that when program heated to withinthe orientation temperature of the polymer, measurable shrinkage whichis neither excessive nor inconsiderable will occur, the range withinwhich it should be controlled by the heat input from the programmedheaters being established from yields of good quality containers 34having minimum thickness variability and the necessary levels ofthickness in the lower body portions formed by axially and radiallystretching in amounts which do not substantially imbalance the resultingorientation in either the axial or radial direction. In arriving atvalues for such variables as heater temperature settings, oven residencetime, preform thickness levels and the extent of stretching in the moldfor any particle thermoplastic material, tracking just where thematerial of a particular part of the preform ends up during stretchingmay be faciliated by initially physically marking the preforms with agrid pattern and then visually examining such markings and thedistribution of plastic thereat in the finished container. Asexemplified in FIGS. 1 and 5, when the above considerations are appliedto nitrile-based materials, i.e. polymers comprising a major proportionof nitrile-group-containing monomer, the ratio of preform averagethickness to preform body inside surface area should be between 0.005 to0.011 inch⁻¹ which should be controlled on heating to provide betweenabout 4 to 15% and preferably 6 to 15% shrinkage. The stretch parametersas above defined should provide A plus B values of between about 130 to280 but with the proviso that A be at least about 30 and B be at leastabout 100. At these stretch levels for such nitrile-based materials,substantial imbalance in the resulting levels of orientation in onedirection versus the other is avoided.

Nitrile-based preforms according to the invention, and as shown in FIG.1 (the arrowed numbers correspond to various preform weights) satisfythe equation:

    y = 0.247 × 10.sup.-3 (X).sup.-2.068

where:

y = % shrink of the preform during heating to orientation temperatureand is between 4 to 15 and X = average preform thickness/inside preformbody surface area.

The preforms of this invention may be formed by conventional injectionmolding techniques from any molecularly orientable thermoplasticmaterial. Typical of such materials are polymers and copolymers ofstyrene, vinyl halides, olefins at at least one aliphatic mono-1-olefinhaving a maximum of 8 carbon atoms per molecule, and polyesters such aspolyethylene terephthalate. The invention has been found particularlyapplicable to nitrile polymers containing a major proportion of apolymerized nitrile-group-containing monomer, such materials generallycomprising from about 50 to about 90% by weight of nitrile monomerunits, based on the total polymer weight, wherein the weight percent ofnitrile is calculated as acrylonitrile. More particularly, the nitrilepolymers used in this invention will comprise at least one nitrilemonomer having the formula: ##STR1## wherein R is hydrogen, an alkylgroup having 1 to 4 carbon atoms or a halogen. Such compounds includeacrylonitrile, methacrylonitrile, ethacrylonitrile, propacrylonitrile,alpha chloronitrile, etc. as well as mixtures thereof. The mostpreferred nitriles are acrylonitrile and methacrylonitrile and mixturesthereof.

The nitrile compositions generally will contain one or more comonomerscopolymerizable with the nitrile monomers including monovinylidenearomatic hydrocarbon monomers of the formula: ##STR2## wherein R¹ ishydrogen, chlorine or methyl and R² is an aryl group of 6 to 10 carbonatoms and may also contain substituents such as halogen as well as alkylgroups attached to the aromatic nucleus, e.g. styrene, alphamethylstyrene, vinyl toluene, alpha chlorostyrene, ortho chlorostyrene,meta chlorostyrene, para chlorostyrene, ortho methylstyrene, paramethylstyrene, ethyl styrene, isopropyl styrene, dichloro styrene, vinylnaphthalene, etc.

Additional useful comonomers include the lower alpha olefins of from 2to 8 carbon atoms, e.g. ethylene, propylene, isobutylene, butent-1,pentene-1 and their halogen and aliphatic substituted derivatimes, e.g.vinyl chloride, vinylidene chloride, etc; acrylic acid and methacrylicacid and the corresponding acrylate and methacrylate alkyl esterswherein the alkyl group contains from 1 to 4 carbon atoms, e.g. methylacrylate, ethyl acrylate, propyl acrylate, methyl methacrylate, etc.Other comonomers which may be used include vinyl esters such as vinylacetate; and alkyl vinyl ethers wherein the alkyl group contains from 1to 4 carbon atoms such as methyl vinyl ether, ethyl vinyl ether etc. andmixtures of the foregoing.

Additional comonomers useful in the practice of this invention are thosecomonomers which contain a mono- or di-nitrile function. Examples ofthese include methylene glutaronitrile, 2, 4-dicyanobutene-1, vinylidenecyanide, crotonitrile, fumaronitrile, maleonitrile. The preferredcomonomers are the monovinylidene aromatic hydrocarbons, lower alphaolefins and acrylic and methacrylic acid and the corresponding acrylateand methacrylate esters with the monovinylidene aromatic hydrocarbonsbeing more particularly preferred. More specifically preferred arestyrene and alpha methylstyrene. Another preferred composition is thatwherein a terpolymer of nitrile, styrene and vinyl ether is used such asdisclosed in U.S. Patent No. 3,863,014.

Optionally, the high nitrile materials may contain from 0 to about 25%by weight of a synthetic or natural rubber component such aspolybutadiene, isoprene, neoprene, nitrile rubbers, acrylate rubbers,natural rubbers, acrylonitrile-butadiene copolymers, ethylene-propylenecopolymers, chlorinated rubbers, etc., which is used to strengthen ortoughen the high nitrile packaging materials. This rubbery component maybe incorporated into the polymeric packaging material by any of themethods which are well known to those skilled in the art, e.g., directpolymerization of monomers, grafting the nitrile monomer onto therubbery backbone, polybend of a rubber graft polymer with a matrixpolymer, etc.

The preferred nitrile polymers for container packaging applicationsrequiring excellent oxygen and water vapor barrier properties are thosecontaining from about 55 to about 85% by weight, based on the totalpolymer weight, of an acrylonitrile and/or methacrylonitrile monomer(wherein the weight percent of methacrylonitrile is calculated asacrylonitrile). When acrylonitrile is used as the sole nitrile monomerthe preferred range is from about 60 to 83% by weight whereas withmethacrylonitrile the preferred range is from about 70 to about 98% byweight of methacrylonitrile which corresponds to about 55 to about 78%by weight of nitrile monomer calculated as acrylonitrile.

The following examples are given to illustrate the principles andpractice of this invention and should not be construed as limitationsthereof.

EXAMPLE I

It was decided to form containers in the form of bottles 34 in FIG. 4from reheated injection molded preforms. Such containers, intended formultiple use applications were to have 950 cc. nominal capacity, aweight of 85 gms., a maximum outside diameter (D in FIG. 4) of 8.3 cms.,a total length of 27.9 cms. and a finish length (33 in FIG. 4) of 1.68cms.

Thermoplastic material in the form of a polymer comprising a 70/30percent mixture by weight of polymerized acrylonitrile/styrene monomerwas injection molded in conventional equipment into preforms configuredas in FIG. 2 having the following dimensional characteristics:

total length = 19.6 cms.; finish length = 1.68 cms.

outside diameter (at 20 in FIG. 2) = 3.20 cms.

average thickness = 0.399 cms.

1/4° taper along preformd body (θ in FIG. 2)

inside body surface area (i.e. exlcuding that of finish 14 in FIG. 2) =123 sq. cms. ##EQU4## With cavity dimensions of a blow mold set toprovide the above bottle configuration, the percentages of axial andradial stretch were calculated at 47% and 159% respectively as follows:##EQU5## These individual levels of A and B are within the cross hatchedarea of FIG. 5 with the total being within the previously determinedacceptable range of 130 to 280.

While rotating about their lengthwise axes, the body portions of suchpreforms (12 in FIG. 2) were heated from substantially room temperatureto within about 132°-138° C. which is the molecular orientationtemperature of the polymer composition of the preforms, in accordancewith a heating arrangement as shown in FIG. 3 wherein the temperature ofthe emitters for the various zones within an enclosing oven were set asfollows:

    ______________________________________                                        Zone     1      2      3    4    5    6    7    8                             ______________________________________                                        Temperature                                                                            422    383    390  413  409  418  408  327                           (° C.)                                                                 ______________________________________                                    

Residence time before the heaters was approximately 196 seconds followedby a conditioning time in air at about 82° C. of 98 seconds to permitthe temperature through the preform walls to equilibrate. Representativepreform samples on exiting the oven were checked for reduction in lengthfrom shrinkage due to strain relaxation and found after appropriateinitial manipulation of the controls on the electrical power to theemitter strips to be about 6.65% of the total initial preform length.The remaining preforms after conditioning were introduced to astretchblow assembly as illustrated in FIG. 4 which included a moldcavity having a surface corresponding in shape and extent to that of thedesired end bottle configuration. The preforms were then stretchedaxially against the base of such cavity and expanded radially againstthe side walls to form the bottle shape.

Bottles thus formed were presented to a thickness measuring instrumentmanufactured by American Glass Research, Inc., of Butler, Penn., Model2697-9-0062. This instrument was preset to reject bottles having athickness in chime area 45 in FIG. 4 either below or above or whichvaried circumferentially beyond certain limits. These settings were 38mils for minimum thickness and 70 mils for maximum thickness. Thicknessvariability was incorporated as a range based on the minimum thicknesssetting and was allowed to vary between 15% at the minimum setting of 38mils to 30% at maximum setting value. These instrument setting valueswere obtained by measuring the performance of calibration bottles notmade according to the present Example but which were determined by thevalues of (a) fill level drop, (b) lean from the vertical, (c) internalpressure strength and (d) impact resistance, to be acceptable as withinestablished specifications for these properties, whereupon settings ofthe thickness monitroing instrument were determined which woulddiscriminate in terms of chime thickness and variability levels betweenbottles equivalent to the calibration bottles and those which were not.

Such tests were as follows:

(a) Fill Level Drop -- Bottles were filled with a carbonated colabeverage at 3.9 volumes CO₂ to a level of 3.5 cms. below the topmostsurface of the finish, then capped and placed in an oven at 37.8° C. for24 hours whereupon they were removed and allowed to return to roomtemperature. The unopened bottles were placed on a flat surface and thenew fill level measured with the difference from the initial level beingthe actual fill level drop. The specification on maxium fill level dropwas 3.8 cms. after exposure to the conditions noted.

(b) Lean -- Each bottle was filled with a carbonated cola beverage at3.9 volumes CO₂, capped and placed in an oven at 37.8° C. for 24 hours,removed and allowed to return to room temperture. The unopened bottleswere placed on a flat, level surface and a dial gauge positionedadjacent each one, such gauge having a feeler resting against the bottlesurface immediately beneath the finish designed to deflect with anydeviation of such surface from vertical, and to indicate the magnitudeof such deflection via a pointer on a face calibration in cms. Eachbottle was then rotated 360° and the total difference between minimumand maximum pointer readings measure, the specification being no greaterthan 1.14 cms.

(c) Impact Resistance -- Filled and capped bottles at room temperaturewere dropped once from a height of 1.0 meter at a 30° angle to thevertical onto a flat steel plate and the number passing noted, thespecification being at least 50% of those dropped surviving withoutrupture.

(d) Burst Pressure -- Bottles filled with tap water were clamped inplace in an Americal Glass Research Incremental Pressure Tester and theinternal pressure gradually increased until each bottle failed. Pressureat failure was noted, the specification on minimum pressure retentionbeing 10.6 kg./cm.²

When bottles made according to this Example were examined by thethickness instrument with the aforementioned settings, it was found that97.8% of those tested were passed as acceptable.

EXAMPLE II

The procedure of Example I was repeated in forming the same bottleconfiguration and size from preforms having the same length, diameterand taper dimensions except that preform weight was reduced to 58 gms.,which resulted in values for average thickness of 0.298 cm. and for theratio of average thickness to body inside surface area of 0.0056 inch⁻¹or 0.0022 cm.⁻¹ The heaters in the reheat oven were set as follows:

    ______________________________________                                        Zone     1      2      3    4    5    6    7    8                             ______________________________________                                        Temperature                                                                            416.7  417    433  452  438  421  457  458                           (° C.)                                                                 ______________________________________                                    

Residence time before the heaters was 120 seconds with conditioning timebeing about 60 seconds. On exiting the oven, shrink of the preforms wasmeasured at about 13%.

Minimum and maximum values on the thickness measuring instrument werethe same as for Example I but since the bottles of this Example wereintended for single trip use in comparison with those of Example I, themaximum percentage variation was preset at 45%.

Of the bottles fabricated and presented to the thickness measuringinstrument preset as stated, 94.5% were passed as acceptable.

EXAMPLE III

The procedure of Example I was repeated except that bottle size wasproportionately reduced from 950 cc. to 475 cc. with the overallconfiguration being otherwise the same. This resulted in a bottle with amaximum diameter of 6.70 cms., a height of 21.6 cms., a weight of 39.5gms., and a finish length of 1.42 cms. The dimensions of the preformsselected to be formed into such containers were as follows: total length16.0 cms.; finish length 1.42 cms.; outside diameter (at location 20)2.53 cms.; average thickness 0.333 cms; inside surface area 85.0 sq.cms.; average thickness/surface area 0.0099 inch⁻¹ or 0.0039 cm.⁻¹ ;axial stretch 38.3%; radial stretch 164.5%, total stretch 202.8%. Theheaters in the oven were set as follows:

    ______________________________________                                        Zone     1      2      3    4    5    6    7    8                             ______________________________________                                        Temperature                                                                            765    617    645  748  670  705  620  555                           (° C.)                                                                 ______________________________________                                    

Residence time before the heaters was about 104 seconds withconditioning time of 52 seconds. On exiting the oven preform shrink wasmeasured at 9%.

When bottles made from the preforms just described were examined todetermine if they were within the thickness specifications of Example Iit was found that 95% of those examined were acceptable.

The following Examples IV and V are provided for comparison purposes toillustrate the poor yield of acceptable bottles obtained when notoperating in accordance with the invention.

EXAMPLE IV

The bottles had the same overall configuration as those in Example Iexcept that maximum outside diameter D was 8.1 cms., the height was 26.7cm., the weight was 49 gms., and the finish length was 1.68 cms. Thepreform selected weight 49 gms.; the total length before heating was19.6 cms.; outside diameter (at 20) was 2.90 cms.; average thickness was0.243 cms.; inside surface area was 138 sq. cms.; average thickness/bodysurface was 0.0045inch⁻¹ or 0.0018 cm.⁻¹ ; axial stretch was 40%; radialstretch was 172%; and total axial plus radial stretch was 212%. Heatersettings in the reheat oven were:

    ______________________________________                                        Zone     1      2      3    4    5    6    7    8                             ______________________________________                                        Temperature                                                                            394    347    363  365  329  329  333  338                           (° C.)                                                                 ______________________________________                                    

Residence time before the heaters as preset above was 112 seconds with56 seconds conditioning time. On exiting the oven preform shrinkage wasdetermined to be 21%. On presentation of the resulting bottles to thethickness measuring instrument, set as in Example II, a yield of 56% ofacceptable bottles was obtained. Such poor yield is believed due toexcessive shrinkage during reheat which resulted in crossover ofmaterial from one preform zone intended for treatment by heaters of onetemperature into adjacent heater zones set at different temperatures.Such poor temperature distribution at the time of remolding resulted inpoor thickness distribution in the lower body portions of the resultingcontainers and low yields.

EXAMPLE V

The procedure of Example I is repeated except that preform dimensionsare as follows: weight 65 gms, length 15.8 cms., outside diameter (at20) 2.54 cms., average thickness 0.422 cms., body inside surface area87.8 sq. cms. average thickness/inside body surface area 0.0122 inch⁻¹,or 0.0048 cm.⁻¹, axial stretch 77.6%, radial stretch 219% and totalstretch stretch 296.6%.

When these preforms are heated via temperature programming with heatersettings generally in accordance with those in Example I and withsomewhat longer residence time to allow absorption of more heat by therelatively thick preform wall as reflected by the relatively high 0.0122inch⁻¹ value, it is believed the percent shrinkage on exiting the ovenwill be about 3.8% which is an indication of a low stress level butbecause such preforms are relatively thick and short, the percent axialand radial stretch amounts to form the bottle are excessive such thatthe quality control limits on thickness in the lower body portions willbe exceeded. Bottles formed by axially stretching and blowing thepreforms of this Example in the amounts indicated herein on presentationto the thickness measuring instrument set as in Example I are believedto provide yields on the order of 40% acceptable bottles.

Various modifications and alterations will be readily suggested topersons skilled in the art. It is intended, therefore, that thefollowing be considered as exemplary only and that the scope of theinvention be ascertained from the following claims.

What is claimed is:
 1. A method of forming containers from injectionmolded preforms of thermoplastic material which comprises:providingpreforms having predetermined dimensional characteristics such thatshrinkage during heating to orientation temperature is maintainablewithin predetermined limits; heating such preforms to such temperaturethrough temperature programming by exposing the preforms to a pluralityof separate heating zones disposed along the length thereof whilemaintaining shrinkage to between about 4 to 15% of the total initialpreform length; and then axially and radially stretching such heatedpreforms at least about 30% and about 100% respectively whilemaintaining total % axial and radial stretch between about 130 to 280 toform the containers, whereby strength is developed and thicknessvariability minimized.
 2. In the method of forming containers frominjection molded preforms comprising a major proportion of a polymerizednitrile-group-containing monomer, which method includes heating thepreforms to molecular orientation temperature through temperatureprogramming by exposing the preforms to a plurality of separate heatingzones disposed along the length thereof followed by axial and radialstretching to container form in a mold,the improvement, thereinproviding reduced thickness variability in lower portions of thecontainers which comprises, in combination, the steps of: controllingshrinkage of the preforms during said heating to between about 4 to 15%of the total initial preform length; and then controlling saidstretching according to the relation ##EQU6## wherein: A is at leastabout 30; B is at least about 100; and A plus B is between about 130 to280.
 3. The process of claim 2 wherein prior to heating the ratio:##EQU7## is between about 0.005 to about 0.011 inch⁻¹.
 4. The process ofclaim 2 wherein the preform comprises from about 50 to about 90 weightpercent of a polymerized monomer selected from the group consisting ofacrylonitrile, methacrylonitirle and mixtures thereof.
 5. The process ofclaim 4 wherein the polymerized monomer is acrylonitrile.
 6. The processof claim 5 wherein the preform comprises a 70/30 weight percent mixtureof polymerized acrylonitrile/styrene.
 7. In the method of formingcontainers from tubular, injection molded preforms comprising a majorproportion of polymerized nitrile-group-containing monomer, which methodincludes heating the preforms to molecular orientation temperaturethrough temperature programming by exposing the preforms to a pluralityof separate heating zones disposed along the length thereof followed byaxial and radial stretching in a mold to form the containers, theimprovement which comprises:(a) providing preforms prior to heatingwherein the ratio of average preform thickness to inside preform surfacearea is between about 0.005 to about 0.011 inch⁻¹ ; (b) controlling theamount of shrinkage of the preforms of step (a) during such heating tobetween about 4 to 15% of the total initial preform length; and then (c)controlling said stretching according to the relation: ##EQU8## wherein:A is at least about 30; B is at least about 100; and A plus B is betweenabout 130 to 280.